GSG2 is a protein kinase that regulates chromosome and spindle function during mitosis and meiosis. GSG2 expression is detected in fetal liver, skin, kidney, small intestine and in all proliferating cells. GSG2 phosphorylates H3 thr3 (H3T3ph) in human cell lines and depletion ofGSG2 by RNA interference reveals that GSG2 is required for H3 thr3 phosphorylation in mitotic cells.